1. Field of the Invention
This invention relates to a brightness signal/color signal separating filter, and more particularly to an improvement thereof which separates brightness signal and color signal from a composite television signal, of e.g. NTSC (National Television System Committee) type of PAL (Phase Alternate Line) type, in dependence upon the magnitude of detected vertical or horizontal correlative energy of the composite television signal.
2. Description of the Related Art
FIG. 16 of the accompanying drawings is a block diagram showing an example of conventional brightness signal/color signal separating filter. In FIG. 16, the numerals respectively designate: 111, an input terminal for inputting NTSC type composite color television signal: 2, an A/D converter for converting an analog composite color television signal into a digital signal; 3 and 4, a first and a second one line delay circuit; 5, compensating delay circuit; 6, a vertical filter; 7, a band pass filter; 8 and 10, output terminals; and 9, substracting circuit.
In operation, the composite color television signal inputted to the input terminal 111 enters the A/D converter 2, the output digital signals of which are supplied to both the first one line delay circuit 3 and the vertical filter 6.
One output of the first one line delay circuit 3 is directly supplied to the vertical filter 6, while another output is delayed by one line in the second one line delay circuit 4 and thereafter supplied to the vertical filter 6.
The vertical filter 6 is generally referred to as a two line type comb-shaped filter, the output of which is fed to the band pass filter 7.
The band pass filter 7 outputs color signals 205, one to the output terminal 8, and another to one input terminal of the subtracting circuit 9. The other input terminal of the subtracting circuit 9 receives the output of the first one line delay circuit 3 through a compensating delay circuit 5 which compensates the amount delayed in the band pass filter 7. The subtracting circuit 9 then outputs a brightness signal 207 to the output terminal 10.
It will now be explained how the above-mentioned filter functions on the NTSC type composite color television signal. The input signal 201 of the composite color television signal having been simultaneously sampled by the color subcarrier with the sampling frequency fs=4 * fsc (fsc: a color subcarrier frequency which is approximately 3.58 MHz in NTSC type, while approximately 4.43 MHz in PAL type) would be displayed in two-dimentional arrangement on the screen as shown in FIG. 17.
Namely, with fsc=(455/2) * fH (fH: horizontal frequency: approximately 15.7 kHz in NTSC and 15.6 kHz in PAL), the phase of the color signal C inverts by 180 degrees per line, resulting in 4 samples at each cycle. Here, assuming that the codes Y and C1, C2 designate brightness signal and color signals respectively in the drawing, the white circle, the slashed circle, the white triangle, and the slashed triangle represent Y+C1, Y-C1, Y+C2, and Y-C2, respectively.
When Z.sup.-1 by Z-conversion is used as a code for representing the delay by one sample, it would be expressed as Z.sup.-1 =exp (-j 2.pi.f/4 fsc). Then, a delay by one line Z.sup.-l becomes Z.sup.-l =exp (-j 2.pi.f/fH)=exp (-j 2.pi.fl/4 fsc), resulting in l=910 upon fsc=4 * (455/2) * FH.
At this time, the vertical filter 3 extracts a line aiding signal 204, for aiding per each line including the color signal, from the one line delay signal and the two line delay signal which are delayed by the first and second one line delay circuit 3 and 4, and from the current input signal 201. The transmission function Hv (Z) of the vertical filter 6 can be expressed as Hv (Z)=(-1/4) * (1-Z.sup.-l)2.
In other words, the line aiding signal Hc (m, n) at the coordinates (m, n) on the screen shown in FIG. 17 is sampled as Hc (m, n)=-(1/4) * {S (m, n-1)-2S (m, n)+S (m, n+1)}. Since the line aiding signal contains the brightness signal Y too, the color signal C (m, n) being high frequency component is separated from the line aiding signal Hc (m, n) by the band pass filter 7. The resulting color signal 205 will be supplied to the substracting circuit 9.
In this case, the transmission function of the band pass filter 7 can be composed, for example, as Hh (Z)=(-1/32) * (1-Z.sup.-2).sup.2 * (1+Z.sup.-4).sup.2 * (1+Z.sup.-8).
The one line delay signal 202 is delayed, in accordance with the delaying amount of the band pass filter 7, by the compensating delay circuit 5. The subtracting circuit 9 substracts the color signal C (m, n) from the signal S (m, n), and thereby extracts the brightness signal Y (m, n) as Y (m, n)=S (m, n)-C (m, n).
As described above, in the conventional brightness signal/color signal separating filter, the characteristics of the vertical filter and the horizontal filter have been fixedly combined. Namely, the brightness signal/color signal separating process has been performed, in both the vertical and the horizontal directions, by a band pass filter.
Consequently, in regions where the brightness and the color of the image greatly change, the brightness signal and the color signal have mutually leaked into the other's channel, causing a degraded reproductive image by e.g. dot interference.